Anything USB/FireWire that is OHCI or EHCI compliant will work in OS 9 and X.

Other cards that don't require boot-time operation (Ethernet, capture cards, specialized accelerators, what have you) can generally work in both PCs and Macs provided that drivers for the operating system exist. Cards that do need boot-time operation (bootable SCSI, IDE, video, etc) require Mac drivers in ROM or firmware (such as Open Firmware) on the card. Typically, these cards are not interchangeable between operating systems without flashing.

Peace,

Drew